The xylenes, para-xylene, meta-xylene and ortho-xylene, are important intermediates which find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but amounts to only 20-25% of a typical C.sub.8 -aromatics stream. Adjustment of isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non-equilibrium mixture of the xylene isomers which is lean in the desired xylene isomer to a mixture which approaches equilibrium concentrations.
Various catalysts and processes have been developed to effect xylene isomerization. In selecting appropriate technology, it is desirable to run the isomerization process as close to equilibrium as practical in order to maximize the para-xylene yield; however, associated with this is a greater cyclic C.sub.8 loss due to side reactions. The approach to equilibrium that is used is an optimized compromise between high C.sub.8 cyclic loss at high conversion (i.e. very close approach to equilibrium) and high utility costs due to the large recycle rate of unconverted C.sub.8 aromatics.
Catalysts for isomerization of C.sub.8 aromatics ordinarily are classified by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but it normally is converted in the isomerization unit because separation from the xylenes by superfractionation or adsorption is very expensive. One approach is to react the ethylbenzene to form a xylene mixture via conversion to and reconversion from naphthenes in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function. An alternative widely used approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. The former approach enhances xylene yield by forming xylenes from ethylbenzene, while the latter approach commonly results in higher ethylbenzene conversion, thus lowering the quantity of recycle to the para-xylene recovery unit and concomitant processing costs.
In the past twenty years or so, crystalline aluminosilicate zeolite-containing catalysts have become prominent for xylene isomerization. U.S. Pat. No. 3,856,872 (Morrison), for example, teaches xylene isomerization and ethylbenzene conversion with a catalyst containing ZSM-5, -12, or -21 zeolite. U.S. Pat. No. 4,626,609 (Shihabi) discloses conversion of xylene isomers using a catalyst comprising a composite which has been steamed at 200.degree. to 500.degree. C. U.S. Pat. No. 4,899,012 (Sachtler et al.) teaches the use of a catalyst containing lead, a Group VIII metal, a pentasil zeolite and an inorganic-oxide binder to isomerize xylenes and dealkylate ethylbenzene. Development efforts continue toward realizing economically attractive isomerization catalysts with a superior combination of activity, selectivity and stability.